Entropy-stabilized transition metal diborides for high-temperature applications

Abstract: Ultra-high temperature ceramics (UHTCs) are typically defined as materials with melting temperatures > 2500 °C, high chemical stability, and retained mechanical resistance at temperatures higher than 1650 °C. UHTCs include carbides and diborides of the Group IVB, VB, and VIB transition metals and are therefore used as structural or protective materials in extreme environments. The introduction of entropy-stabilization into the ceramics field has sparked interest regarding their properties across a diverse range of UHTC compositions. Entropy plays a dominant role in the formation of single-phase multicomponent ceramics, offering new pathways for synthesis and enabling property optimization without relying on hazardous or costly elements. Entropy-stabilized materials have a highly disordered and homogeneous chemical composition, and these materials have been demonstrating enhanced and tunable properties when compared to traditional materials. A series of entropy-stabilized and high-entropy ceramics materials as diborides, carbides, oxides, nitrides, etc. have been rapidly developed. In this thesis, by screening potential high-entropy ceramic candidates via ab initio calculations considering both boron and carbon occupying the anion sublattice, six high-entropy ceramics compositions containing Li, Ti, V, Zr, Nb, and Hf were initially proposed. Subsequently, we have focused and covered the design, synthesis, and high-temperature oxidation and ablation properties of the entropy-stabilized (Ti0.25V0.25Zr0.25Hf0.25)B2. The diboride was sintered as bulk and as coating on graphite by Spark Plasma Sintering (SPS) and as bulk by Ultra-fast High-temperature Sintering (UHS). Dual-phase and single-phase entropy-stabilized (Ti0.25V0.25Zr0.25Hf0.25)B2 were obtained by SPS. The dual-phase diboride was a metastable phase composed of Hf-Zr-rich and Ti-V-rich hexagonal phases that transformed into a single-phase entropy-stabilized diboride after thermal annealing. Examination of the oxidation behavior of both the dual-phase and single-phase diborides indicated that at 1000 °C, a surface B2O3 layer was formed, following a para-linear oxidation kinetics. At 1500 °C, a porous oxide layer was created, enabling oxygen diffusion and further oxidation of the diboride, leading to linear oxidation kinetics. The entropy-stabilized diboride outperformed the dual-phase diboride in oxidation resistance terms, attributed to the high-entropy and sluggish diffusion effects. The entropy-stabilized (Ti0.25V0.25Zr0.25Hf0.25)B2 was also manufactured as a coating on graphite and tested under ablation at 2200 °C. The mechanical stability of the coating at high temperatures was attributed to the efficient heat dissipation of the coating-graphite pair, and to the coating’s low thermal conductivity (< 6.52 W m-1 K-1). Mechanical denudation and evaporation of B2O3 and light V-Ti oxides were identified as the primary ablation mechanisms. The manufacturing by UHS enabled the investigation of pressure-less and fast sintering of the (Hf0.25Zr0.25Ti0.25V0.25)B2-B4C composite using different currents. The entropy-stabilized phase formation happened before full densification of the composite and the mechanical properties of the fully densified composite are comparable to the pure entropy-stabilized diboride with similar density produced by SPS.Overall, the results obtained by this work contribute to the growing body of knowledge surrounding entropy-stabilized ceramics, their design and fabrication through computational and experimental methods, and their potential applications in engineering components. These findings open many new paths to be followed in the entropy-stabilized materials realm.